As a radiation detector, especially alpha rays detector, for example, a detector that uses a ZnS scintillator is known. The ZnS scintillator emits light at a time when alpha rays enter. On the other hand, an alpha ray observation device is known that makes use of the characteristics of alpha rays that cause nitrogen in the atmosphere to emit light. This device is able to observe alpha rays even from a remote place by making use of the characteristics of alpha rays that cause nitrogen in the atmosphere to emit light and observing the light emitted from nitrogen in order to detect alpha rays. The light being emitted is ultraviolet light.
FIG. 9 is a lateral cross-sectional view showing an example of a conventional configuration of an alpha ray observation device that observes the light emitted from nitrogen to detect alpha rays.
As shown in FIG. 9, what is known is an alpha ray observation device that includes: a collecting lens 101, which collects the light emitted from nitrogen; a wavelength selection element 102, which extracts, from the collected light, the light emitted from nitrogen; an optical element 103, which separates the extracted nitrogen-originated light into transmitted light and reflected light; a direction changing unit 104, which changes the propagation direction of the reflected light; light detectors 105a and 105b, which respectively receive the transmitted light and the reflected light to count the number of photons; and a signal processing device 106, which selects the nitrogen-originated light caused by alpha rays as the light detector 105a measures the transmitted light and the light detector 105b measures the reflected light simultaneously.